1. Field of the Invention
This invention is in the field of traffic control systems in which signals from a traffic controller are applied to load switches to cause the load switches to energize traffic signal lights to regulate the flow of potentially conflicting vehicular traffic. More particularly, this invention relates to circuits for detecting the status of one group of traffic signal lights which control automobile traffic on one highway (non-intersecting traffic generally referred to as a single phase of traffic), for producing an error or fault signal if the lights of such group overlap for more than a predetermined period of time or if none of them are energized for more than a predetermined period of time, and for identifying the group of lights the status of which produced the error signal.
2. Description of the Prior Art
The use of traffic signal lights to control the flow of traffic, particularly at the intersection of two or more streets or highways, is well known. The traffic signal lights controlling traffic at one intersection are typically controlled by a local traffic controller which is programmed to produce control signals which are applied to load switches. The load switches in turn energize and de-energize, or turn on and turn off, the traffic signal lights of the system. Typically, one load switch controls one or more traffic lights which are connected in parallel with a source of electrical power, normally AC. The lights controlling the flow of traffic on one street (single phase traffic) are grouped together physically and logically. As a typical minimum, such a group of lights would consist of a single traffic signal head with a minimum of one red, one yellow and one green signal light for controlling traffic coming from a single direction, and would most likely include a second red, yellow and green signal light coupled in parallel with the first red, yellow and green signal light for controlling traffic coming from an opposite direction. Obviously, other types of lights such as left turn, right turn, etc., can also be included and if they are, there would be a separate load switch for each such type. In the typical minimum configuration, one load switch would be associated with each of the red, yellow and green lights of a single traffic head and the load switches for one signal head would constitute a group or block, of such switches.
The prior art has recognized that problems can arise in the operation of traffic control systems such as the simultaneous energization of the green signal lights of two intersecting streets which could cause accidents in the intersection instead of preventing accidents, assuming drivers would, or could, always obey the signals.
In U.S. Pat. No. 3,629,802 issued Dec. 21, 1971, entitled, CONFLICTING PHASE ERROR DETECTOR, by L. K. Clark et al., command signals produced by the local traffic controller which are applied to load switches are also applied to a logic circuit. When the occurrence of coincident command signals is detected which could cause conflicting traffic movement, an error signal is produced which is normally employed to cause all the yellow or red signals, or a combination thereof, to flash on and off.
In U.S. Pat. No. 3,648,233 which issued on Mar. 7, 1972 and is entitled, LOAD CONTROL ERROR DETECTOR, by L. K. Clark, the problem solved is the possibility of traffic signal lights of a traffic control system being energized in the absence of a command signal being applied to the load switches for energizing a traffic signal light. An error detector is connected to the controller and to a conductor between a traffic signal light and its load switch, which conductor is a part of the circuit for energizing the signal light. The error detector produces an error signal if the traffic signal light is energized and no control signal is supplied to the corresponding load switch. An error signal when produced will cause the traffic control system to assume a flashing mode until someone services the system to eliminate the problem.
The relevant prior art has been primarily concerned with detecting malfunctions of the traffic controller which could potentially result in conflicting command signals being issued by the traffic controller and the energization of a traffic signal when no command for that light to be energized has been applied to the load switch controlling the energization of the signal light.
The prior art does not recognize or address the problem of detecting certain malfunctions that can cause the traffic signal lights controlled by one group of load switches to overlap; i.e., for two or more different colored signal lights controlling the flow of a single phase of traffic on one street, for example, to both be energized for a period of time greater than that specified for the yellow and green lights to overlap. Another problem that is not recognized or solved is the failure of a signal light to be energized at any time during operation because, for example, a signal light has failed, a load switch has failed, the circuit linking the signal light to the load switch has failed, or the source of the electrical energy may have failed.
Another problem the prior art does not recognize is that of how to quickly identify the source of the failure of the traffic control system. Finding the cause of a failure can be difficult and time consuming, particularly when encountered with a complex traffic control system that is operating in a flashing mode. To minimize this problem of identifying the cause of the problem, applicant's invention provides a visual signal that will identify to the serviceman the group of traffic signal lights and their associated load switches that were the source of the error.